1. Field of the Invention
The present invention is directed to automatic circuit testers and, in particular, to an improved manufacturing defect analyzer (MDA) having a hybrid scanner.
2. Related Art
For reasonably comprehensive testing of circuit boards, the number of circuit-board test points ("nodes") that test instruments must be connected at one time or another is typically quite large. But the number of points that have to be connected at any one time is relatively a smaller fraction of the total. The test instruments can therefore be multiplexed. The term used in this art for the multiplexing hardware is "scanner."
Some manufacturing defect analyzer (MDA) circuit testers have used semiconductor switches to implement the scanner. Because of the accuracy costs that such switches' high impedances exact, however, they have been employed only in a few testers directed to a very limited region of the fault spectrum. By and large, more comprehensive testers use mechanical relays instead. With such scanners, considerable in-circuit measurement accuracy can be achieved. In addition, some circuit testers perform measurements on powered-up digital circuits, where it is sometimes necessary to force large currents through a pin channel. Semiconductor switches that carry high currents have high capacitance associated with them, making them unsuitable for use in scanners. Semiconductor switches having suitable capacitance values, can only carry a few tens of milliamperes without risking damage, are therefore also unsuitable. As a result, tester manufacturers have borne the significant burdens that mechanical relays impose, such as limited reliability, low probe-addressing flexibility, high relay-energization power requirements, the need to provide mechanical reinforcement to support the relays' added weight, and the much greater space required by the relays, their drivers, and the resultant power supplies.
What is needed, therefore, is a circuit tester scanner that can support tests for identifying defects covering a large portion of the fault spectrum without incurring the above drawbacks of conventional scanners.